We have had very good results with the GPRSbee module from SODAQ. It’s a 2G board and SODAQ also has a new 3G board with the same Bee footprint so it could also work with the Mayfly. We have deployed many stations with the GBRSbee board along with a SIM card from a company that offers a low-cost data plan, so the loggers can push data into our custom online database every 5 minutes. They work anywhere that there’s a decent AT&T or T-Mobile signal. Verizon uses different hardware and also doesn’t permit devices like this on their network anyway.
We plan to offer kits later this month that include a GPRSbee and SIM card, along with publishing a tutorial for how to build the logger stations that use the Xbee and/or cellular modules.